The best performing, commercially available AlInGaN light emitting devices (LEDs) are grown on an insulating substrate, e.g. sapphire. Electrodes and their connection pads are usually placed on top of the AlInGaN semiconductor layers of the device.
During operation, current is injected into the LED through external terminals that are tied to the connection pads by wire-bonding (ball or wedge), soldering, or attaching with a conducting adhesive. The p- and n-electrodes inject and spread the current into the respective semiconductor layers. Light is generated when current flows across the p-n junction in the forward direction which causes the recombination of minority carriers at the p-n junction. The intensity, I, of the light emitted by the device under typical operating conditions is proportional to the current density, J, the current per unit area. For a given current density, J, the larger the area of the p-n junction, the greater the intensity, I, generated by the LED.
The p-type semiconductor layers in the AlInGaN materials system are much more resistive than the n-type semiconductor layers. As a result, the current injected at the p-electrode does not spread laterally within the p-type semiconductor and does not spread laterally away from the p-electrode. The current flows from the p-electrode along the shortest path (i.e., usually vertically) across the p-n junction to the n-type semiconductor layers. The current then spreads laterally within the n-type semiconductor layers to reach the n-electrode.
To maximize the area of optical emission, the current must flow across as much of the p-n junction as possible. Consequently, the current must be spread laterally over as large a fraction of the p-type surface as possible. Lateral current spreading may be improved by covering most of the p-type surface with the p-electrode. The p-electrode is then covered totally or in part with a connection pad.
Connection pads are conducting to provide their electrical functionality and must be thick to meet their mechanical functionality. As a result, connection pads are usually metallic. Metal connection pads of the required thickness are opaque. Bonding pads made of transparent, conductive oxides such as ITO (Indium Tim Oxide) have also been used, although not commonly.
A large fraction of the commercially available AlInGaN LEDs extract the light generated within the device through the p-layer. These devices have a compound p-electrode, e.g. a thin, semi-transparent material, for current-spreading that covers most of the p-surface, and a thick opaque connection pad that covers as little of the thin p-electrode as possible while still providing reliable connections for commercial manufacture. An n-electrode is made small as well, to maximize the p-type surface area. A large fraction of the optical emission generated at the p-n junction escapes the device through the portion of the semitransparent p-electrode that is not blocked by the connection pad.
In U.S. Pat. No. 5,563,422, Nakamura, et al., teaches that the n and p connection pads should be diametrically opposed or located at the corners of the device, as shown in FIG. 1. The current flowing vertically down to the n-layer from the area of the p-electrode next to the p-connection pad has to traverse a large horizontal distance in the n-type semiconductor layer to reach the n-electrode once it has passed vertically through the p-n junction. On the other hand, the current flowing vertically down to the n-layer from the area of the p-electrode next to the n-connection pad has to traverse a small horizontal distance in the n-type semiconductor layer to reach the n-electrode. The larger distance adds a significant amount of series resistance in the n-type layer to the former current path, resulting in current crowding at the edge of the thin p-electrode around the n-contact. The most direct current path between the two connection pads is favored strongly over any other paths (such as the ones following the edges of the device), causing the current to crowd in between the connection pads. The non-uniformity in current density increases as the average current density increases, due to the increasing resistive voltage drop in the n-type semiconductor layer. This non-uniformity in current density causes corresponding non-uniformity in light intensity, as shown in FIG. 2. The degree of current density non-uniformity is indicated by a ratio r of the maximum local current density J.sub.max to the average current density J.sub.ave. To estimate this ratio r, one can measure the ratio R of the maximum local light intensity I.sub.max to the average light intensity I.sub.ave, since to first approximation the intensity is proportional to the current density. Such measurements are commonly made using an optical apparatus and imaging the biased LED in near-field conditions. As can be seen from FIG. 2, the ratio R is very high.
The non-uniformity in current density leads to a reduction in the optical and electrical performance of the LED, particularly for conditions of high average current density, and for LEDs that are made with larger dimensions. AlInGaN LEDs characteristically exhibit decreasing efficiency of light emission as the average current density increases due to the mechanism of light emission. Hence, non-uniformity in the current density results in regions operating with lower overall optical efficiency. In addition, as the irreversible degradation in efficiency of light emission increases with increasing current density, non-uniformity in current density increases the overall rate of degradation, a significant concern for commercial LEDs that rely on low rates of degradation for their commercial importance.
An additional drawback to the prior art is that the electrode configurations inefficiently use the substrate area as light-emitting material. For a given average current density J, the larger the area of the p-n junction, the greater the average light intensity I generated by the LED.